Yup as Kevin says some racing seat mounts offer attachment for the OEM seat belt receptacle. You can utilize the factory 3-pt system this way.

As for a 6-pt system, the only safe and proper way of installing them is to attach the shoulder belts to the roll cage/roll bar, and the anti-sub and lap straps to the chassis of the car with proper, welded reinforcement plates. Don't let anyone convince you otherwise.

Here are some pictures from the GT4 as they came from BMW. Courtesy of CheckCaptain.